Fixing devices employing a heat roller fixing method are widely used as fixing devices for use in electrophotographic image forming apparatuses such as copy machines and printers. The fixing device employing the heat roller fixing method includes (i) a pair of rollers (a fixing roller and a pressure roller) which press each other and (ii) heating means made of, e.g., a halogen heater, the heating means being disposed within both or either one of the pair of the rollers. The heating means heats the pair of rollers to a predetermined temperature (a fixing temperature). After that, a recording paper sheet on which an unfixed toner image is formed is carried to a pressure area (a fixing nip area) between the pair of rollers, and then the recording paper sheet is caused to pass through the pressure area. Thus, the unfixed toner image is fixed on the recording paper sheet due to heat and pressure applied thereto.
Incidentally, a fixing device included in a color image forming apparatus generally uses an elastic roller, which is a fixing roller provided with, on its surface, an elastic layer made from silicon rubber or the like. In the case where the elastic roller is used as the fixing roller, a surface of the fixing roller elastically deforms according to an uneven surface of an unfixed toner image and comes in contact with the toner image so as to cover the toner image. This makes it possible to favorably fix, by using heat, a color unfixed toner image whose toner amount is larger than that of a monochrome unfixed toner image. Further, due to strain release of the elastic layer which occurs in the fixing nip area, it is possible to improve releasing property with respect to color toner, which is more likely to offset than monochrome toner. Furthermore, the fixing nip area has a nip shape protruding upward (i.e., toward the fixing roller side), that is, a so-called inverse nip shape. This makes it possible to more favorably separate a recording paper sheet from the fixing roller, thereby allowing the recording paper sheet to be separated without use of any separation means such as a separation claw (self-stripping). This prevents insufficient image formation which is caused by the separation means.
Now, in order that the fixing apparatus included in such the color image forming apparatus responds to a higher process speed, the fixing nip area needs to have a greater nip width. Examples of means for increasing the nip width encompass: a method of increasing a thickness of the elastic layer of the fixing roller; and a method of increasing a diameter of the fixing roller.
However, a fixing roller having a thicker elastic layer causes the following problem: The elastic layer has a low heat conductivity. Therefore, in a case where the heating means is provided within the fixing roller having the thicker elastic layer as in the conventional fixing devices, increasing a process speed results in insufficient heat supply, so that a temperature of the fixing roller does not follow the increased process speed. Meanwhile, increasing the diameter of the fixing roller reduces curvatures of the rollers forming the fixing nip area, thereby increasing the fixing nip area. Increasing the diameter of the fixing roller, however, requires the rollers to increase their heat capacities, thereby causing problems of an extended warm-up time and increased electric power consumption.
In order to solve these problems, recently, as the fixing device included in the color image forming apparatus, Patent Literature 1 discloses a fixing device employing an external belt heating fixing method that uses an external heating belt to heat a fixing roller from the outside, for example. The fixing device employing the external belt heating fixing method can efficiently heat the fixing roller from the outside, thereby reducing a warm-up time. Further, for example, Patent Literature 2 discloses a fixing device (belt fixing device) employing a belt fixing type. In the belt fixing device, a heating roller, which is heating means, is provided outside a fixing roller, a fixing belt is set between the fixing roller and the heating roller, and the fixing roller and a pressure roller press each other via the fixing belt. In the belt fixing device, the fixing belt having a small heat capacity is heated. This can reduce a warm-up time. Further, with any of these fixing methods, it is not necessary to incorporate a heating source such as a halogen lamp into the fixing roller. This makes it possible to increase a thickness of a low-hardness elastic layer made from sponge rubber and/or the like, thereby securing a large nip width.
Furthermore, Patent Literatures 3 and 4, etc. disclose fixing devices (planar heat-generating belt fixing devices) each employing a planar heat-generating belt fixing method. Each of the planar heat-generating belt fixing devices is a belt fixing device provided with a planar heater which curves along heating means. In the planar heat-generating belt fixing device, the heating means has a smaller heating capacity than those of conventional heating rollers. Further, in the planar heat-generating belt fixing device, the planar heater, serving as the heating means, directly generates heat. Thus, compared with the conventional belt fixing methods in which the heat roller is indirectly heated with use of the halogen lamp, the planar heat-generating belt fixing device has improved thermal response, and enables further reduction in a warm-up time and further energy saving.